MFC r351408-r351410: reduce pollution from mips machine/regnum.h

r351408:
libsa: mips: use _JB_* from machine/asm.h, remove regnum dep

This brings the libsa/mips _setjmp implementation closer to parity with the
libc version.

r351409:
mips: hide regnum definitions behind _KERNEL/_WANT_MIPS_REGNUM

machine/regnum.h ends up being included by sys/procfs.h and sys/ptrace.h via
machine/reg.h. Many of the regnum definitions are too short and too generic
to be exposing to any userland application including one of these two
headers. Moreover, these actively cause build failures in googletest
(template <typename T1 ...> expanding to template <typename 9 ...>).

Hide the definitions behind _KERNEL or _WANT_MIPS_REGNUM, and patch all of
the userland consumers to define as needed.

r351410:
libsa: mips: fix typo that had slipped into the diff on local machine